The invention relates to the field of converting heat energy to mechanical energy utilizing a working fluid, particularly for, but not necessarily limited to generating electricity.
In order to perform useful work, energy must be changed in form, i.e., from potential to kinetic, heat to mechanical, mechanical to electrical, electrical to mechanical, etc. The experimentally demonstrated equivalence of all forms of energy led to the generalization of the first law of thermodynamics, that energy cannot be created or destroyed, but is always conserved in one form or another. Thus, in transforming energy from one form to another, one seeks to increase the efficiency of the process to maximize the production of the desired form of energy, while minimizing energy losses in other forms.
Mechanical, electrical and kinetic energy are energy forms which can be transformed into each other with a very high degree of efficiency. This is not the case, however, for heat energy; if we try to transform heat energy at a temperature T into mechanical work, the efficiency of the process is limited to 1-T.sub.0 /T, in which T.sub.0 is the ambient temperature. This useful energy which can be transformed is called exergy, while the forms of energy which cannot be transformed into exergy are called anergy. Accordingly, the first law of thermodynamics can be restated that the sum of exergy and anergy is always constant.
Moreover, the second law of thermodynamics which states that processes proceed in a certain defined direction and not in the reverse direction, can be restated that it is impossible to transform anergy into exergy.
Thermodynamic processes may be divided into the irreversible and the reversible. In irreversible processes, the work done is zero, exergy being transformed into anergy. In reversible processes, the greatest possible work is done.
Energy conversion efforts are based upon the second law, to make the maximum use of exergy before it is transformed into anergy, a form of energy which can no longer be used. In other words, conditions must be created to maintain the reversibility of processes as long as possible.
The present invention is concerned with the conversion of heat energy to mechanical energy, particularly for the generation of electrical power, the process which presents the greatest problems with regard to efficiency. In the processes, heat is transferred to a working fluid which undergoes a series of temperature, pressure and volume variations in a reversible cycle. The ideal regenerative cycle is known as the Carnot cycle, but a number of other conventional cycles may be used, especially the Rankine cycle, but also including the Atkinson cycle, the Ericsson cycle, the Brayton cycle, the Diesel cycle and the Lenoir cycle. Utilizing any of these cycles, a working fluid in gaseous form is passed to a device for converting the energy of the working fluid to mechanical energy, which devices include turbines as well as a wide variety of other types of heat engines. In each case, as the working fluid does useful mechanical work, the volume of the fluid increases and its temperature and pressure decrease. The remainder of the cycle is concerned with increasing the temperature and pressure of the working fluid so that it may perform further useful mechanical work. FIGS. 1A-1J give P-V and T-S diagrams for a number of typical cycles.
Since the working fluid is an important part of the cycle for doing useful work, a number of processes are known in which working fluid is modified in order to increase the work that can be obtained from the process. For example, U.S. Pat. No. 4,439,988 discloses a Rankine cycle utilizing an ejector for injecting gaseous working fluid into a turbine. By utilizing the ejector to inject a light gas into the working fluid, after the working fluid has been heated and vaporized the turbine was found to extract the available energy with a smaller pressure drop than would be required with only a primary working fluid and there is a substantial drop in temperature of the working fluid, enabling operation of the turbine in a low temperature environment. The light gas which is used can be hydrogen, helium, nitrogen, air, water vapor or an organic compound having a molecular weight less than the working fluid.
U.S. Pat. No. 4,196,594 discloses the injection of a rare gas, such as argon or helium, into a gaseous working fluid such as aqueous steam used to carry out mechanical work in a heat engine. The vapor added has a lower H value than the working fluid, the H value being C.sub.p /C.sub.v, C.sub.p being specific heat at constant pressure and C.sub.v being specific heat at constant volume.
U.S. Pat. No. 4,876,855 discloses a working fluid for a Rankine cycle power plant comprising a polar compound and a non-polar compound, the polar compound having a molecular weight smaller than the molecular weight of the non-polar compound.
In considering the conversion of heat energy to mechanical energy, an extremely important thermodynamic property is enthalpy. Enthalpy is the sum of the internal energy and the product of pressure and volume, H=U+PV. Enthalpy per unit mass is the sum of the internal energy and the product of the pressure and specific volume, h=u+Pv. As pressure approaches zero, all gases approach the ideal gas and the change of the internal energy is the product of the specific heat, C.sub.p0 and the change of temperature dT. The change of "ideal" enthalpy is the product of C.sub.p0 and the change of temperature, dh=C.sub.p0 dT. When pressure is above zero, the change of enthalpy represents the "actual" enthalpy.
The difference between the ideal enthalpy and the actual enthalpy divided by the critical temperature of the working fluid is known as residual enthalpy.
Applicant has theorized that greater efficiency from a reversible process is feasible if one can increase the change in actual enthalpy of a system, within the range of temperature and pressure conditions as required by its previous design. This could conceivably be accomplished by methods which would result in the release of "residual" enthalpy, in effect, slowing down the loss of exergy in the system.
Another extremely important property of a working fluid is the compressibility factor Z, which relates the behavior of a real gas to the behavior of an ideal gas. The behavior of an ideal gas under varying conditions of pressure (P), volume (V) and temperature (T), is given by the equation of state: EQU PV=nMRT
where n is the number of moles of gas, M is the molecular weight, and R is R/M, where R is a constant. This equation does not actually describe the behavior of real gases, where it has been found that: EQU PV=ZnMRT or Pv=ZRT
where Z is the compressibility factor, and v is specific volume ##EQU1## For an ideal gas Z equals 1, and for a real gas, the compressibility factor varies depending upon pressure and temperature. While the compressibility factors for various gases appear to be different, it has been found that compressibility factors are substantially constant when they are determined as functions of the same reduced temperature and the same reduced pressure. Reduced temperature is T/Tc, the ratio of temperature to critical temperature and reduced pressure is P/Pc, the ratio of pressure to critical pressure. The critical temperature and pressure are the temperature and pressure at which the meniscus between the liquid and gaseous phases of the substance disappears, and the substance forms a single, continuous, fluid phase.
Applicant has also theorized that a greater volumetric expansion could be obtained by modifying the compressibility factor of a working fluid.
Applicant has further theorized that substance could be found which would increase both the enthalpy and compressibility of a working fluid.